1. Field of the Invention
This invention relates to a radio receiver which receives at least a Frequency Modulation (FM) broadcast signal and demodulates the FM broadcast signal by digital signal processing.
2. Description of Related Art
Recently, digitization of signal processing of an FM/AM car-radio receiver has become popular with getting high-performance and high efficiency of multipath compliant and electrical noise rejection, and improving disturbing features by removing adjacent channel disturbing. For example, techniques or products related to radio reception by digital signal processing are described in “TDA7580” Data Sheet, STMicroelectronics, 2007 (non patent document 1), “SAF7730HV”, Philips Semiconductors, Car Stereo Designer's Guide 2004/2005, order # 9397 750 13849, p. 29, September 2004 (non patent document 2), and “Symphony™ Digital Radio Introduction”, Product Preview, p. 18 and 26, Freescale Semiconductor, Inc., SYMPHONYCHPSET/D Rev. 0, January 2003 (non patent document 3).
The product “TDA7580” of STMicroelectronics Company described in the non patent document 1 is a digital signal processer (DSP) in which an analog IF signal converted into IF (Intermediate Frequency) band from RF (Radio Frequency) band by external analog front end (AFE) module is received and performs digital demodulation is performed by digital sampling. That is, in the product described in the non patent document 1, a digital circuit (TDA7580) which performs digital demodulation of the IF signal is isolated from an analog circuit (AFE module) which performs RF-IF conversion.
Further, the DSP product “TDA7580” described in the non patent document 1 incorporates an oscillation circuit which oscillates in synchronization with an external crystal oscillator of 74.1 MHz, and generates an operation clock for digital signal processing and supplies a reference clock (for FM: 100 kHz, for Europe AM: 18 kHz) to AFE module.
The product “SAF7730HV” of Philips Semiconductors Corporation described in the non patent document 2 is an in-car DSP. The DSP described in the non patent document 2 as well as the one described in the above-described the non patent document 1 in which an analog IF signal supplied by external AEF module is input performs digital demodulation by digital sampling. Therefore, in the product described in the non patent document 2, a digital circuit (SAF7730HV) which performs digital demodulation of IF signal is isolated from an analog circuit which performs RF-IF conversion.
Further, the DSP product “SAF7730HV” described in the non patent document 2 incorporates an oscillation circuit which oscillates in synchronization with an external crystal oscillator of 41.6 MHz, and generates an operation clock for digital signal processing.
The product of Freescale Semiconductor Corporation described in the non patent document 3 includes an RF front end IC “SDR510100”, which converts FM/AM broadcast signal into an analog IF signal by RF-IF conversion, and outputs the analog IF signal, and an IF analog interface IC “SDR530100”, which receives the analog IF signal, and performs digital demodulation processing by digital sampling. Therefore, in the product described in the non patent document 3, a digital circuit (IF analog interface IC) which performs digital demodulation of IF signal is isolated from an analog circuit (RF front end IC) which performs RF-IF conversion.
Further, the IF analog interface IC “SDR530100” described in the non patent document 3 incorporates an oscillation circuit which oscillates in synchronization with an external crystal oscillator of 28.8 MHz, and generates an operation clock for digital signal processing and supplies a reference clock (7.2 MHz) to RF front end IC “SDR510100”.
Japanese Unexamined Patent Application Publication No. 9-93149 (patent document 1) discloses the technique which restrains a poor reception when FM airwaves are received according to an FM multiplex broadcasting receiver. FIG. 7 is a block diagram showing an FM multiplex broadcasting receiver described in the patent document 1. In FIG. 7, many parts of an FM receiver 1 are constructed by an analog circuit and the FM receiver 1 receives FM airwaves and demodulates an FM multiplex broadcast signal which is a multiple digital signal. A detection part 2 detects existence or nonexistence of receiving of FM airwaves from the FM receiver 1. The FM multiplex broadcast signal which is obtained by demodulation processing of the FM receiver 1 is input into a main controller 3, and the main controller 3 performs error correction, displays information processing for a display device, and so on. A clock controller 4 generates a clock signal supplied to the main controller 3. Further, the clock controller 4 decreases a frequency of a clock signal corresponding to the detection result of the detection-part 2 when the FM receiver 1 receives the FM airwaves. This restrains a poor reception of FM airwaves due to an interference of the clock signal.
The patent document 1 takes note that, although the main controller 3 which is a digital circuit does not operate very fast, the main controller 3 can receive the digital signal of FM multiplex broadcasting because the transfer rate of the digital signal is low (for example, 16 kbit/s).
The receiver which demodulates an FM broadcast signal by digital signal processing has a problem that an electromagnetic wave which radiates from the digital circuit becomes an interference wave corresponding to the FM broadcast signal of high-frequency wave, by the frequency of an operation clock of the digital circuit which performs digital signal processing or harmonic frequency thereof being superimposed to FM broadcast band. Not only direct radiation from an oscillation circuit which supplies a clock signal to the digital circuit but also generation of current synchronized with the clock signal, specifically, radiation of an electrical field which is due to the current flowing between a power supply (VDD) and a ground (GND) often become the interference wave.
The case that the receiver receives the FM broadcast signal will be described. The FM broadcast band of all over the world is the following: from 76.0 MHz to 90.0 MHz in Japan, from 87.5 MHz to 108.0 MHz in Europe, from 87.9 MHz to 107.9 MHz in North America, and from 65.0 MHz to 74.0 MHz in East Europe. Therefore, in the case where the operating frequency of the digital circuit or the harmonic frequency thereof are superimposed to the FM broadcast bands, a tremendous interference is possibly generated for the reception of the FM broadcast wave of a specific frequency.
As described above, controlling for decreasing the frequency of the clock signal of the digital circuit when the receiver receives the FM multiplex broadcasting is described in the patent document 1. However, in the case where the digital signal processing is applied to the reception of the signal (FM broadcast signal) of an analog broadcast of an FM radio, lowering the frequency of the clock signal leads to a decrease in performance of the signal processing. That is, it would be substantially impossible to achieve the performance because a reception processing of the FM broadcast signal may not be performed by decreasing the frequency of the clock signal.
In the case of the non patent document 1, as the operating frequency of the digital circuit is 74.1 MHz, there is only a difference of 100 kHz between the operating frequency and an upper limit frequency of 74.0 MHz of an FM broadcast band of East Europe. Therefore, an electromagnetic wave radiated from the digital circuit may become the interference when the receiver receives the FM broadcast signal of 74.0 MHz.
In the case of the non patent document 2, as the operating frequency of the digital circuit is 41.6 MHz, the frequency of 83.2 MHz of the secondary harmonic wave of the operating clock is superimposed to the FM broadcast band of Japan. Therefore, an electromagnetic wave radiated from the digital circuit may become an interference when the receiver receives the FM broadcast signal in the proximity of 83.2 MHz.
In the case of the non patent document 3, as the operating frequency of the digital circuit is 28.8 MHz, the frequency of 86.4 MHz of the third harmonic wave of the operating clock is superimposed to the FM broadcast band of Japan. Therefore, an electromagnetic wave radiated from the digital circuit may become the interference when the receiver receives the FM broadcast signal in the proximity of 86.4 MHz.
As described above, the products which demodulate the FM broadcast signal by the digital signal processing cannot avoid interference which is due to the operating frequency of the digital circuit itself or harmonic frequency being superimposed to the FM broadcast band. Therefore, in general, to prevent certainly that an electromagnetic wave from the digital modulator restrains to interfere in the AFE module, the radio receiver employs the construction that the AFE module is arranged away from the digital modulator and an IC including the digital demodulator or an IC including the AFE module or both of them are covered with a shielded member which blocks an electromagnetic wave.
For the purpose of reference, FIG. 8 shows a configuration example of an in-car audio equipment for car mounted with a conventional radio receiver. In FIG. 8, a signal which has an RF band and is received by an antenna 100 is input into an RF-IF converter 101. Then the RF-IF converter 101 selects a desired channel by adjusting a local frequency generated by a frequency synthesizer (not shown), and converts the selected signal into an analog IF signal of 10.7 MHz, for example, and outputs it. In the example of FIG. 8, a reference frequency of the frequency synthesizer which the RF-IF converter 101 includes is supplied from an IF demodulator 107 which is described later.
The IF demodulator 107 takes a digital sample of an analog IF signal which is supplied from the RF-IF converter 101, performs a digital demodulation processing, and outputs a stereo audio signal (an L signal and an R signal) Further, the IF demodulator 107 has an oscillation circuit (not shown) which oscillates in synchronization with an oscillator (an OSC for radio) 108 for a radio receiver connected to the IF demodulator 107, and performs the digital signal processing using a clock signal output by the oscillation circuit, or a multiplied clock of it, as an operation clock. Furthermore, the IF demodulator 107 generates a reference clock signal (reference frequency f_REF) by dividing an output of an oscillation circuit (not shown) synchronized with the OSC for radio 108 and supplies the reference clock signal to the frequency synthesizer which the RF-IF converter 101 includes.
An audio signal processor 110 is a so-called “DSP”, which performs a digital audio signal processing such as a correction of volume level or frequency equalizing with respect to a digital audio signal supplied from the IF demodulator 107, a semiconductor memory processor 113, an optical disk processor 114, and so on. The audio signal which is processed by the audio signal processor 110 is digital-analog converted and is supplied to an amplifier 112, and is output from a speaker 118.
An OSC for audio 111 is an oscillator for digital audio signal processing. The audio signal processor 110 has an oscillator circuit (not shown) synchronized with the oscillator for audio (OSC for audio) 111, and performs a digital audio signal processing using the clock signal which the oscillation circuit outputs, or the multiplied clock of it, as an operation clock.
The semiconductor memory processor 113 reads out an audio file stored in a semiconductor memory 131, and supplies the digital audio signal to the audio signal processor 110.
An optical disk processor 114 reads out a digital audio signal stored in an optical disk 132, and supplies it to the audio signal processor 110.
A main controller 115 performs complete control of the in-car audio equipment. An oscillator for the system (OSC for system) 116 is an oscillator which supplies an operation clock to the main controller 115.
A display controller 117 controls a display of a display device 133. Frequency of an FM broadcast channel currently being tuned is shown in the display device 133, for example.
In the in-car audio device of FIG. 8, the AFE part including the RF-IF converter 101 is implemented on a sub substrate 122, and a digital circuit part which is a latter part of IF demodulator 107 is implemented on a main substrate 123 which is outside the sub substrate 122. Further, connecting the sub substrate 122 and a digital signal processing module of the main substrate 123 after making the sub substrate 122 implemented as the AFE part to be a tuner module having a metal shield structure, can prevent a clock signal generated by three kinds of oscillators (OSC for radio 108, OSC for audio 111, and OSC for system 116) described above from influencing a high-frequency circuit which is a latter part of an antenna 100, that is, the RF-IF converter 101.